1. Field of the Invention
The present invention relates to a method for cutting a hollow cylindrical workpiece and a cutting apparatus for carrying out the method.
2. Description of Related Art
Rotary cutting is known as a way to cut a hollow cylindrical workpiece to obtain a certain shape. For the rotary cutting, a button tip (hereinafter, called a tip) is mounted at a distal end of a tool mounted on a head of a processing apparatus. The rotary cutting is a method for cutting the workpiece by rotating the workpiece and the tool (tip) while the tip is being fed (refer, for example, to Japanese Patent Application Publication No. 2006-68831 (JP 2006-68831 A)).
As illustrated in FIG. 9, in the conventional rotary cutting, when a rotational center line Cx of a workpiece 90 is horizontal, the direction of an axis line C1 of a linear tool 99 is set to a direction inclined by 30 degrees or smaller (A 30 degrees) with respect to the vertical direction. The feed direction of the tool 99 (tip 98) is an arrow Y1 direction (direction close to a radial direction of the tip 98) in FIG. 9. The axis line of the tip 98 coincides with the axis line C1 of the tool 99. The tip 98 is rotated by rotation of the tool 99 about the axis line C1. In the case of the rotary cutting, since the tip 98 rotates, the life of the tip 98 is longer and the processing efficiency is higher than in the case of ordinary single point processing.
As illustrated in FIG. 9, the hollow cylindrical workpiece 90 is held at an end portion 91 in an axial direction thereof by a chucking device 95. When the cutting begins, a large cutting resistance is generated between the workpiece 90 and the tool 99. This cutting resistance is large in an arrow X direction (tangential direction of the workpiece 90) in FIG. 9. Accordingly, the chucking device 95 needs to firmly hold the workpiece 90. That is, the chucking device 95 illustrated in FIG. 9 is configured to hold the hollow cylindrical workpiece 90 by catching it from a radially inner side of the workpiece 90. The direction of supporting the workpiece 90 by holding it in this manner is the same as (parallel to) the direction (arrow X direction) of the above-described cutting resistance acting on the workpiece 90. A large holding force is therefore required for the chucking device 95 to resist the large cutting resistance. In particular, the cutting resistance further increases if the workpiece 90 has high hardness after heat treatment. The holding force of the chucking device 95 only needs to be increased according to the cutting resistance. In this case, however, the workpiece 90 is distorted, and deteriorates in accuracy after the processing. The rotary cutting is therefore generally considered unsuitable for finishing.
If the workpiece 90 is hollow cylindrical, the conventional rotary cutting method as illustrated in FIG. 9 can be employed for cutting an outer circumference of the workpiece 90. However, this method cannot be directly employed for cutting an inner circumference. That is, feeding the tool 99 (tip 98) in the arrow Y1 direction causes the tool 99 to interfere with the workpiece 90, so that the cutting cannot be performed. To prevent the interference, as illustrated in FIG. 10, a protruding length L of the tool 99 only needs to be increased. In this case, however, the tool 99 decreases in stiffness to make the cutting difficult. As described above, due to the arrangement relation between the tool 99 and the workpiece 90, the rotary cutting is used for cutting the outer circumference of the workpiece 90.